1. Field of the Invention
The subject invention pertains to conductive-fiber reinforced composites, and hybrid structures of metals and such composites. More particularly, the subject invention pertains to methods for reducing galvanic degradation of such composites and galvanic corrosion of metals which are electrically connected thereto.
2. Description of the Related Art
Galvanic corrosion of metals is a thoroughly researched and relatively well-understood phenomenon. The presence of two metals of differing electrochemical potential or the presence within one metal part of demes having different electrochemical potential (as, for example, occurs in carbon rich zones in iron and carbon steel) will result in the creation of a galvanic cell when exposed to oxygen and an electrolyte solution, resulting in rapid corrosion.
In order to prevent galvanic corrosion, one of the necessary components of the galvanic cell must be eliminated or its effectiveness somehow thwarted. One of several solutions or redundant combinations thereof are commonly used. One such method is to introduce a third, more electrochemically active (sacrificial) metal which will itself predominately corrode. The use of a more active sacrificial metal such as magnesium or zinc has been utilized in gross structure, and on specialized applications such as ocean-going vessels and power boats, the latter to protect aluminum outdrives. Use of sacrificial coatings, ingots and the like, however, adds unnecessary weight, must be frequently monitored and replaced as necessary, and is not effective for portions of the structure which are or may become electrically isolated. Moreover, when the corrosible metal is itself an active metal, a more electrochemically active metal may not exist or lend itself to fabrication.
A second, more common method of reducing galvanic corrosion is to paint the metal surface with a coating which is impervious to oxygen and/or electrolyte solution. Unfortunately, no known polymeric coating is completely water or oxygen impervious. Moreover, inadequate coverage, particularly around edges and other irregularly shaped surfaces, and defects in the coatings caused by scratching, collision, flexing of the part, etc., can rapidly decrease the ability of such coatings to provide protection.
A third method of protection, in concert with the second, is to add to the paint or coating corrosion inhibitors which function to coat, through chemical reaction, with a more impervious coating than the paint itself, or which slowly migrate to the metal surface, and which interfere with the cathodic or anodic reactions which take place in the metal couple. Examples of corrosion inhibitors are lead compounds, which are no longer used due to toxicity and environmental concerns, and chromates, which are affected by these same concerns but to a lesser degree.
These and other methods of reducing corrosion are discussed in Chemical Inhibitors for Corrosion Control, Proc. of the Int. Symp. Organized by the Industrial Div. of the Royal Society of Chemistry and the Institution of Corrosion Science and Technology, University of Manchester, Apr. 21-22, 1988, B. G. Clubley, Ed.; and H. Leidkeiser, Jr., "Corrosion of Painted Metals--A Review," appearing in Corrosion, National Association of Corrosion Engineers, Vol. 38, No. 7, July 1982, pp. 374-382.
Fiber reinforced, polymer matrix composites are being increasingly used in the aerospace and transportation industries. These products offer exceptional strength and rigidity while at the same time allowing weight savings over metal counterparts. As the toughness of such composite materials has increased, so has their utilization. In many structures, however, it is necessary to attach such composite structures to metal parts. If the reinforcing fibers are non-conductive, such as glass, quartz or Spectra.RTM. polyolefin fibers, then such "hybrid" structures function well for extended periods of time, even in aggressive environments. In the early 1980's, however, it was discovered that when hybrid structures were prepared using conductive carbon fiber-reinforced epoxy resin composites, that galvanic corrosion of the attached metal may occur, the conductive fibers serving as the cathode in the galvanic cell. With aluminum substrates, currents on the order of milliamps may be measured between the exposed carbon fibers and the aluminum. See S. D. Thompson, B. L. White and J. A. Snide, Accelerated Corrosion Testing of Graphite/Epoxy Composites and Aluminum Alloy Mechanically-Fastened Joints, Flight Dynamics Laboratory, Air Force Wright Aeronautical Laboratories, report AFWAL-JR-84-3115. Suggested methods for combatting such corrosion were the same methods traditionally utilized for metal/metal galvanic corrosion--isolation of substrates from one another, or from oxygen or electrolyte, by painting, in particular painting the machined edge of the composite where carbon fibers are directly exposed to the environment. It was, however, noticed that voids in the paint were not uncommon, and corrosion would be expected to be accelerated at such locations due to trapping of moisture (electrolyte) at these locations. Thus, galvanic corrosion of metals in electrical contact with carbon fibers was still viewed as a potential problem despite the use of protective measures.
This perceived problem was enhanced with the discovery that not only did hybrid structures cause galvanic corrosion of metals but, moreover, degradation of the polymer matrix of the composite could occur if the polymer matrix is susceptible to base hydrolysis. As the majority of composite parts contained epoxy-based matrix resins which are relatively immune from such attack, this phenomenon had gone unnoticed. At the 36th International SAMPE Symposium, Apr. 15-18, 1991, two papers were presented which directed attention to composite degradation. In "Galvanic Corrosion Effects on Carbon Fiber Composites," J. Boyd et al., pp. 1217-1231; and "Relationship of Graphite/Polyimide Composites to Galvanic Processes," M. D. Faudree, pp. 1288-1301, evidence was presented that composites containing carbon fiber reinforced bismaleimide polymer matrices themselves degraded in addition to promoting corrosion of aluminum. The degradation was clearly shown by actual loss of polymer matrix as well as increasingly exposed amounts of fibers.
This effect is illustrated in FIG. 1, in which a galvanic cell is created when a corrosible metal (such as aluminum) is electrically connected to a conductive-fiber (such as carbon) reinforced composite. The metal serves as the anode while the conductive-fibers serve as the cathode. The anode reaction results in dissolution of the metal thusly: EQU Al+3Cl.fwdarw.AlCl.sub.3 +3e.sup.-,
while the reaction at the cathode may be reviewed as: EQU 4e.sup.- +O.sub.2 +Na.sup.+ +2H.sub.2 O.fwdarw.4NaOH, or EQU 2e.sup.- +2H.sub.2 O+2Na.sup.+ .fwdarw.2NaOH+H.sub.2.
Although not wishing to be bound by any particular theory, it is believed that the creation of hydroxide ions is responsible for polymer degradation through base hydrolysis or resin fiber debonding.
Among the solutions to the corrosion/degradation problem proposed by the aforementioned articles include traditional methods of protection, via paints or corrosion inhibiting primers, or by use of non-conductive scrim layers between the composite and metal to provide electrical insulation. Unfortunately, these methods add cost and weight to the structure, and are not viewed by airframe manufacturers as entirely satisfactory. In addition, because of the sensitivity of air frame manufacturers in particular to any loss in strength or other physical properties of aircraft parts which have the potential of corroding or degrading galvanically, there has been some resistance to use hybrid structures of composites containing conductive fibers and a polymer matrix susceptible to degradation.
It is, therefore, an object of the subject invention to provide processes and compositions for use therein which offer the potential of reducing galvanic action in both a galvanically corrosible metal as well as a galvanically degradable polymer matrix composite when these are contained in hybrid metal/composite structures.